1. Field of the Invention
This invention relates to diaphragm devices, and more particularly to a lens barrel diaphragm device of the type in which a diaphragm mechanism arranged in the interchangeable lens is driven by a stepping motor.
2. Description of the Prior Art
The 35 mm cameras of the non-interchangeable lens type are generally called compact cameras. In a camera of this type, as the exposure device, there is installed a lens shutter device which also serves as a diaphragm. The lens shutter device installed in the recently developed compact camera employs a direct type electromagnetic drive structure. In such a lens shutter device, the sector ring which bears the shutter blades (or diaphragm blades) functions itself as the rotor of the electromagnetic drive device (that is, motor). Also, though not put into practice, another type of lens shutter device in which the sector ring is rotated by a stepping motor of cylindrical shape (or ring shape) having a ring-shaped rotor of as large a diameter as the lens barrel is proposed. (See, for example, Japanese Laid-Open patent application No. Sho 58-17428).
In these prior known lens shutter devices, because the sector ring which bears the diaphragm blades (sectors) is driven by the motor directly connected thereto, there is no need to use a mechanism of complicated structure for rotating the sector ring to an arbitrary angle and setting a location. Therefore, they are small in size and light in weight, and are suited to electronic control.
Meanwhile, in the single lens reflex camera whose lens has a far larger diameter than in the lens shutter camera, and is interchangeable (hereinafter referred to as the "reflex camera"), there is employed such a layout that, within the camera body, a focal plane shutter is installed, while, within the lens barrel (or interchangeable lens), a diaphragm device is mounted. This diaphragm device is arranged so as to be mechanically driven from a diaphragm drive device provided in the camera body through a linkage lever. In the conventional reflex camera of such structure, not only is the mechanical structure of the diaphragm drive device in the camera body complicated, but also other mechanisms in the camera body which relate to the diaphragm drive device including the linkage lever, too become complicated. Therefore, there is a drawback in that the weight of the entire camera including the lens barrel is heavy and the volume of the entire camera is large. Also, since the transmitting of the driving force to the diaphragm device is performed through a complicated mechanical power transmission intermediary, the responsibility of the control operation is bad, and it is not suited to electronic control.
For this reason, in the conventional reflex camera, by improving the diaphragm device, the weight of the whole of the camera can be reduced. Thus, it is required to realize a diaphragm device of good control response characteristics.
In order to realize a diaphragm device which enables the weight and volume of the camera body and interchangeable lens to be reduced, and the control response characteristics to be improved, it is desirable that the diaphragm device of the reflex camera is also constructed as a direct motor driven type diaphragm device.
However, in the past, the electromagnetic drive device for the lens shutter mounted in the compact camera as has been described before, had its sector ring (or a rotary plate of the same size as the sector ring) of the diaphragm blades itself constitute the rotor, and, in the lens shutter device disclosed in the above-cited document, a ring of large diameter constituted the rotor. Therefore, the inertia of the rotor itself is very large. As is known in the art, letting the diameter of the rotating body be denoted by D and the gravity acceleration by G, the inertial mass of the rotating body is proportional to GD.sup.2. Hence, in the above-described electromagnetic drive device whose rotor is a ring-like body of a large value in D, the inertial mass of the rotor is very large. Therefore, there were drawbacks that the starting characteristics and the control response characteristics were poor also, since the coil for the stator was formed on a ring with its center at the optical axis, and because the number of turns of the coil could not be increased, a high ampere turn could not be obtained, and, therefore, a small starting torque could only be obtained. Therefore, although it was usable as the diaphragm drive motor for the lens of small diameter as in the compact camera, it was impossible to employ the structure of that electromagnetic drive device as the motor for the diaphragm device of large diameter in the reflex camera. That is, if a motor for the diaphragm device of large diameter is designed to have the same structure as that of the above-described electromagnetic drive device, the magnetomotive force cannot be increased at as high a rate as the inertial mass of the rotor increases. As a result, starting the driving of the rotor becomes difficult, and it will easily fall out of control. So, it will frequently occur that the rotor, while fluctuating in the stop position, does not rotate.